IPT systems are a well known area of established technology (for example, wireless charging of electric toothbrushes) and developing technology (for example, wireless charging of handheld devices on a ‘charging mat’). Typically, a primary side (transmitter) generates a time-varying magnetic field with a transmitting coil or coils. This magnetic field induces an alternating current in a suitable receiving coil in a secondary side (receiver). This induced current in the receiver can then be used to charge a battery, or power a device or other load. In some instances, the transmitting coils or the receiving coils may be connected with capacitors to create a resonant circuit, which can increase power throughput and efficiency at the corresponding resonant frequency.
Typically, receivers used in IPT systems consist of: a pickup circuit (e.g. a resonant circuit in the form of an inductor and capacitor); a rectifier for converting the induced power from AC to DC; and a switched-mode regulator for regulating the voltage of the power ultimately provided to a load.
A common problem with receivers used in IPT systems is that the switched-mode regulator may include a DC inductor. The DC inductor acts as an energy store so that power can be suitably regulated. Such DC inductors can be a bulky circuit component, significantly affecting the total size occupied by the receiver. This can be a particular problem in applications where it is preferable that the receiver be as small as possible (for example, handheld devices).
In some receivers that do not use a DC inductor, complex phase timing is needed to ensure that circuit components are not damaged. This makes sensing and control more complex, and more susceptible to noise and failure.
Another common problem with receivers used in IPT systems is that variations in the operating frequency of the transmitter or resonant frequency of the receiver (due to, for example, changes in load or other circuit parameters), can affect the amount and efficiency of power transfer.
For receivers, it is known to include switches in power converter stages (e.g. switched-mode regulators or synchronous rectifiers). Often, these switches are controlled so that they switch when the voltage across the switch is zero (zero-voltage switching (ZVS)) or the current through the switch is zero (zero-current switching (ZCS)). The benefits of ZVS and ZCS are well-known, including minimising losses in the switches. In order to suitably control the switches to achieve ZVS or ZCS, it is necessary to detect the phase of the voltage or current in the circuit. One known method for detecting the phase is using a current sense resistor. However the current flowing in an IPT receiver can have a large dynamic range, which can in turn lead to the current sense resistor overheating at high loads and unreliable signals at low loads. Another known method for detecting phase is using a current transformer. However such current transformers are bulky and may not be fast enough for high-frequency applications.
It is an object of the invention to provide receivers for IPT systems that minimise the size of the DC inductor.
It is a further object of the invention to provide receivers for IPT systems that do not include a DC inductor.
It is a further object of the invention to provide a method for detecting phase in a receiver that performs well for a range of loads from low to high, and a method that does not rely on bulky circuit components.
Each object is to be read disjunctively with the object of at least providing the public with a useful choice.